1. Field of the Invention
The present invention relates to a wiper motor driving device for automotive vehicles, and more specifically to a wiper motor driving device suitable for use in snowy weather, by which the so-called "wiper blade hunting operation" (the wiper motor is turned on or off by the reaction force of the wiper blade whenever the wiper motor is turned off) can be prevented.
2. Description of the Prior Art
An ordinary wiper motor for automotive vehicles is designed so as not to stop immediately after the wiper switch has been turned off. Instead, the motor continues to rotate until an automatic wiper stop mechanism stops the wiper blade when the blade reaches a horizontal position on the windshield. This is because the wiper blade should not be stopped in the driver's vision. An example of devices as described above is disclosed in Japanese Published, Unexamined (Kokai) Utility Model Application No. 61-113062, for instance, as shown in FIGS. 1(A) and (B).
In these drawings, the automatic wiper motor stop mechanism includes a wiper motor 1, a worm shaft 2 driven by the wiper motor 1, a worm wheel 3 geared with a worm gear formed in the worm shaft 2 so as to form a reduction gear, a roughly circular grounding plate 4 formed with a sectorial cutout portion, and a sectorical autostop plate 5 located at the cutout portion. Grounding plate 4 and autostop plate 5 are concentrically attached to worm wheel 3. In addition, first and second fixed spring contacts 6a and 6b are positioned so that they are selectively brought into contact with grounding plate 4 and the autostop plate 5, respectively. That is, when the worm wheel 3 rotates, the first fixed contact 6a is brought into contact with or separated from both grounding plate 4 and autostop plate 5, while the second fixed contact 6b is brought into contact with or separated from only autostop plate 5, as depicted in FIG. 1(B),
With reference to FIGS. 2(A), (B) and (C), the operation of the above automatic wiper blade stop mechanism will be explained hereinbelow.
FIG. 2(A) shows a state in which a wiper switch 7 is at the LOW position. Since a closed circuit is formed from the power supply Acc through the wiper motor 1 to the ground E.sub.1, as shown by the arrows, the wiper motor 1 continues to rotate regardless of the positions of the two fixed contacts 6a and 6b (LOW WIPER DRIVE).
Next, if the wiper switch 7 is set to the OFF position, the closed circuit shown by the arrows in FIG. 2(B) is formed. The wiper motor 1 continues to rotate as long as there is a conducting path from the fixed contact 6a through the grounding plate 4, to the ground E.sub.2 (DRIVE AFTER OFF).
When the grounding plate 4 reaches the position as shown in FIG. 2(C), the two fixed spring contacts 6a and 6b both come into contact with the autostop plate 5, and at the same time the circuit shown by the arrow is formed. Therefore, an armature shorting (generator control) circuit can be formed to stop the wiper motor 1 momentarily (STOP BY ARMATURE SHORT).
In this configuration, the automatic wiper blade stop mechanism operates in such a way that when the wiper switch 7 is turned off, the wiper motor 1 always stops at a predetermined position or when the wiper blade reaches a specified position on the windshield of a vehicle.
In the above-mentioned prior-art device, there exists a problem in that the hunting or chattering phenomenon occurs, because grounding plate 4 is rotated in the reverse direction by a wiper blade, in particular when snow .is accumulates on the windshield. Once the above hunting operation occurs, other problems occur in that the worm wheel 3 is overheated and therefore its resin part melts, or the electrical circuit develops trouble.
In more detail, in snowy regions, where the considerable amount of snow often accumulates on the windshield, it is impossible for the wiper blade to perfectly clear away the snow on the windshield and therefore snow tends to be put at the lower side of the windshield. Therefore, when the wiper switch 7 is turned off in snowfall, the wiper blade is often pushed back (upward) by a reaction force due to the snow which has accumulated on the lower side of the windshield, so that worm wheel 3 or the grounding plate 4 is rotated in the reverse direction.
As already described, even after wiper switch 7 has been turned off, the wiper blade is kept rotated in the forward direction F (counterclockwise, for instance) to a predetermined lower position as depicted in FIG. 2B. In this case, there exists a state where both the fixed contacts 6a and 6b ar separated away from the grounding plate 4 as shown in FIG. 1(B) and therefore the wiper motor 1 is not driven nor stopped by shorting the armature. However, in ordinary conditions, since the worm wheel 3 is rotated by the inertia, the two fixed contacts 6a and 6b are both smoothly brought into contact with the autostop plate 5 to short the armature of the motor 1 into the state as shown in FIG. 2(C). In snowy weather, however, since the inertia of the worm wheel 3 is absorbed by snow, there exists the state where the two fixed contacts 6a and 6b are separated from both the grounding plate 4 and the autostop plate 5 as shown in FIG. 1(B). Further, when the wiper blade is pushed back (upward) by the snow accumulated on the lower side of the windshield, since the worm wheel 3 or grounding plate 4 is rotated in the reverse direction R (counterclockwise, for instance) in FIG. 1(B), the fixed contact 6a is again brought into contact with the grounding plate 4, so that the grounding plate 4 is again rotated in the forward direction F, thus resulting in the hunting or chattering operation. Since the above operation is continuously repeated, the wiper motor 1 is started and then stopped repeatedly.
To overcome the above-mentioned problems, another prior-art wiper motor driving device as shown in FIG. 3 has been proposed. In this device, a protrusion 8 is formed on the outer surface of worm wheel 3. A rotating contact wheel 10 concentric with the worm wheel 3 is fitted to a gear cover 9 which covers worm wheel 3. Further, a protrusion 11 contactable with the protrusion 8 on worm wheel 3 is provided on the rear surface 10a of the contact wheel 10. An automatic wiper blade stop mechanism (not shown) for wiper motor 1, which is similar to that shown in FIG. 1(A) (including a grounding plate 4, an autostop plate 5, and two fixed contacts 6a and 6b ) is arranged on the front surface 10b of the contact wheel 10.
In this prior-art device, when worm wheel 3 rotates in the forward direction F, since the protrusion 8 on the worm wheel 3 is brought into contact with the protrusion 11 on the contact wheel 10 to rotate the contact wheel 10 also in the forward direction, the wiper motor 1 can be started or stopped in the same way as explained with reference to FIG. 1(A). In this device, however, since the contact wheel 10 is driven by the worm wheel 3 via the two protrusions 8 and 11, when the wiper switch 7 is turned off the worm wheel 3 is further rotated in the reverse direction by snow accumulated on the lower side of the windshield, the contact wheel 10, that is, the grounding plate 4 will not be rotated in the reverse direction (because only the protrusion 8 is separated away from the protrusion 11), so that it is possible to prevent occurrence of the hunting operation. In this device, however, many parts such as the gear cover 9, the contact wheel 10, the, two protrusions 8 and 11, etc. are additionally required, thus resulting in other problems because the manufacturing cost and the weight are both increased.
On the other hand, Japanese Published Unexamined (Kokai) Patent Application 61,71253 discloses a wiper intermit operation control system which can prevent the above-mentioned hunting operation. This system comprises a wiper motor, an intermittent signal generating circuit, a first switching element turned on or off to drive the motor in response to the intermittent signals, a motor switching element turned on or off in linkage with the motor, a current holding circuit for driving the motor by a predetermined angle, after the intermittent signals have been turned off, on the basis of the turning-on or -off operation of the first and motor switching elements, a second switching element for turning on or off the current holding circuit, and a flip-flop circuit which is set in response to the intermittent signal to turn on the second switching element and reset in response to the on/off signal from the motor switching element to turn off the second switching element.
The above-mentioned prior-art control system is further complicated in its configuration and high cost.